When choosing the appropriate the sampling location in multi-zone drying you must consider what happens during fault conditions.
The first or second zone might normally show the highest solvent concentration. But during an upset, it is not necessarily the zone where the peak concentration will appear. Excess solvent from an upset may very well tend to "carry over" into the next zone and produce a peak concentration there rather than in the first zone.
Therefore the analyzer in the first zone, which is appropriately located and necessary for the normal steady state measurement of solvent concentration, is not always optimally located for the measurement of an upset. To properly detect the upset, an analyzer is needed in a later zone, even though this zone might typically not carry large solvent loads.
Analyses to determine the quantity and positions of analyzers based too heavily on the normal dryer behavior, and not fault conditions, are to be avoided.